application_controller.rb

class ApplicationController < ActionController::Base
  before_filter :set_locale

  protect_from_forgery

  def set_locale
    logger.debug "* Accept-Language: #{request.env['HTTP_ACCEPT_LANGUAGE']}"
    I18n.locale = current_user ? current_user.locale : (params[:locale] || extract_locale_from_accept_language_header || I18n.default_locale)
    logger.debug "* Locale set to '#{I18n.locale}'"
  end

  def default_url_options(options={})
    logger.debug "* default_url_options is passed options: #{options.inspect}\n"
    if current_user
      { :locale => current_user.locale }
    else
      { :locale => I18n.locale }
    end
  end

  private
    def extract_locale_from_accept_language_header
      if request.env['HTTP_ACCEPT_LANGUAGE']
        locale = request.env['HTTP_ACCEPT_LANGUAGE'].scan(/^[a-z]{2}/).first
        locale.match(/^(en|it)$/) ? locale : I18n.default_locale
      else
        I18n.default_locale
      end
    end
end

localization.feature

Feature: Auto Localization
  In order to reach the widest audience for my site
  A visitor of the site
  Should be able to read the content in her/his language

    Scenario: Italian visitor visit the homepage
      Given I am an italian visitor (it;it-IT)
      When I visit the homepage
      Then I should read the content in italian (it)

    Scenario: English visitor visit the homepage
      Given I am an english visitor (en;en-US)
      When I visit the homepage
      Then I should read the content in english (en)

    Scenario: Foreign visitor visit the homepage
      Given I am an svedish visitor (sv;sv-SV)
      When I visit the homepage
      Then I should read the content in italian (it)

localization_steps.rb

Given /^I am an (.*?) visitor \((.*?)\)$/  do |nationality, locale|
  page.driver.header 'Accept-Language', locale
end

When /^I visit the homepage$/ do
  visit root_path
end

Then /^I should read the content in (.*?) \((.*?)\)$/ do |nationality, locale|
  page.should have_content I18n.t("pages.home.claim", :locale => locale)
end

routes.rb

MyApp::Application.routes.draw do
  scope "(:locale)", :locale => /en|it/  do
    # resources
  end
end